Abstract. This paper provides a detailed description of the atmospheric conditions
characterizing the high Himalayas, thanks to continuous observations begun
in March 2006 at the Nepal Climate Observatory-Pyramid (NCO-P) located at
5079 m a.s.l. on the southern foothills of Mt. Everest, in the framework of
ABC-UNEP and SHARE-Ev-K2-CNR projects. The work presents a characterization
of meteorological conditions and air-mass circulation at NCO-P during the
first two years of activity. The mean values of atmospheric pressure,
temperature and wind speed recorded at the site were: 551 hPa, −3.0 °C,
4.7 m s−1, respectively. The highest seasonal values of temperature
(1.7 °C) and relative humidity (94%) were registered during the
monsoon season, which was also characterized by thick clouds, present in
about 80% of the afternoon hours, and by a frequency of cloud-free sky of
less than 10%. The lowest temperature and relative humidity seasonal
values were registered during winter, −6.3 °C and 22%, respectively,
the season being characterised by mainly cloud-free sky conditions and rare
thick clouds. The summer monsoon influenced rain precipitation (seasonal
mean: 237 mm), while wind was dominated by flows from the bottom of the
valley (S–SW) and upper mountain (N–NE).

The influence of the brown cloud (AOD>0.4) extending over Indo–Gangetic
Plains up to the Himalayan foothills has been evaluated by analysing the
in-situ concentrations of the ABC constituents. This analysis revealed that brown
cloud hot spots mainly influence the South Himalayas during the pre-monsoon,
in the presence of very high levels of atmospheric compounds (BC: 1974.1 ng m−3, PM1: 23.5 μg m−3,
scattering coefficient: 57.7 Mm−1, coarse particles: 0.64 cm−3, O3: 69.2 ppbv,
respectively). During this season 20% of the days were characterised by a
strong brown cloud influence during the afternoon, leading to a 5-fold
increased in the BC and PM1 values, in comparison with seasonal means. Our
investigations provide clear evidence that, especially during the
pre-monsoon, the southern side of the high Himalayan valleys represent a
"direct channel" able to transport brown cloud pollutants up to 5000 m a.s.l., where the pristine atmospheric composition can be strongly
influenced.